It is known in the art to use heat treatable aluminum alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. Aluminum alloys AA2024, AA2324 and AA2524 are well known heat treatable aluminum alloys which have useful strength and toughness properties in T3, T39 and T351 tempers. Heat treatment is an important means for enhancing the strength of aluminum alloys. It is known in the art to vary the extent of enhancement by altering the type and amount of alloying constituents present. Copper and magnesium are two important alloying constituents.
The design of a commercial aircraft requires various properties for different types of structures on the aircraft. Especially for fuselage skin or lower wing skin it is necessary to have properties such as good resistance to crack propagation either in the form of fracture toughness or fatigue crack growth. At the same time the strength of the alloy should not be reduced. A rolled alloy product either used as a sheet or as a plate with an improved damage tolerance will improve the safety of the passengers, will reduce the weight of the aircraft and thereby improve the fuel economy which translates to a longer flight range, lower costs and less frequent maintenance intervals.
It is known in the art to have AA2x24 alloy compositions with the following broad chemistry, in weight percent:
Cu3.7-4.4Mg1.2-1.8Mn0.15-0.9 Cr0.05-0.10Si≦0.50Fe≦0.50Zn≦0.25Ti≦0.15
the balance aluminum and incidental impurities.
U.S. Pat. No. 5,593,516 discloses a high damage tolerant Al—Cu alloy with a balanced chemistry comprising essentially the following composition (in weight %):
Cu2.5-5.5Mg0.1-2.3Cumax−0.91 Mg + 5.59Cumin−0.91 Mg + 4.59Zrup to 0.2, orMnup to 0.8
balance aluminum and unavoidable impurities. It also discloses T6 and T8 tempers of such alloys which gives high strength to a rolled product made of such alloy.
U.S. Pat. No. 5,897,720 and U.S. Pat. No. 5,938,867 disclose a high damage tolerant Al—Cu alloy with an “AA2024”-chemistry comprising essentially the following composition (in weight %):
Cu:3.8-4.9Mg:1.2-1.8Mn:0.3-0.9
the balance aluminum and unavoidable impurities wherein the alloy is annealed after hot rolling at a temperature at which the intermetallics do not substantially dissolve. The annealing temperature is between 398° C. and 455° C. U.S. Pat. No. 5,938,867 also discloses an alloy where the ingot is inter-annealed after hot rolling with an anneal temperature of between 385° C. and 468° C.
EP-0473122, as well as U.S. Pat. No. 5,213,639, disclose an aluminum base alloy comprising essentially the following composition (in weight %):
Cu3.8-4.5Mg1.2-1.8Mn0.3-0.9Fe≦0.12Si≦0.10
the remainder aluminum, incidental elements and impurities, wherein such aluminum base is hot rolled, heated and again hot rolled, thereby obtaining good combinations of strength together with high fracture toughness and a low fatigue crack growth rate. More specifically, U.S. Pat. No. 5,213,639 discloses an inter-anneal treatment after hot rolling the cast ingot with a temperature between 479° C. and 524° C. and again hot rolling the inter-annealed alloy. Such alloy appear to show a 5% improvement over the above mentioned conventional 2024-alloy in T-L fracture toughness and an improved fatigue crack growth resistance at certain ΔK-levels.
EP-1045043 describes an aluminum-copper alloy of the general 2024-type which is highly deformable and which comprises essentially the following composition (in weight %):
Cu3.8-4.5Mg1.2-1.5Mn0.3-0.5
the remainder aluminum, incidental elements and impurities, wherein such aluminum alloy is preferably used for sheet applications with gauges in a range of 1.6-5.9 mm. Most examples given are directed to a reduced amount of copper, namely an amount (in weight %) of 3.9-4.2, thereby keeping the amount of magnesium above 1.2.
EP-1026270 discloses another 2024-type aluminum-copper alloy for aeronautical lower wing applications. Such alloy comprises essentially the following composition (in weight %):
Cu3.8-4.4Mg1.0-1.5Mn0.5-0.8Zr 0.08-0.15,
the remainder aluminum, incidental elements and impurities. Such alloy shows an enhanced combination of strength, fatigue crack growth resistance, toughness and corrosion resistance. The alloy may be used for rolled, extruded or forged products wherein the addition of zirconium adds strength to the alloy composition (Rm/Rp (L)>1.25).
EP-A-1114877 discloses another aluminum alloy composition of the AA2xxx-type alloys for fuselage skin and lower wing applications having essentially the following composition (in weight %):
Cu4.6-5.3Mg0.1-0.5Mn0.15-0.45
the remainder aluminum, incidental elements and impurities. The method includes a solution heat treatment, stretching and annealing. Such alloy has been mentioned as being useful for thick plate applications such as wing structures of airplanes. The levels of magnesium are below 0.5 weight % wherein it is disclosed that such low magnesium level is advantageous for age formability. However, it is believed that such low magnesium levels have a negative influence with regard to the alloy's resistance to corrosion, its response to natural aging and its strength level.
U.S. Pat. No. 5,879,475 discloses an age-hardenable magnesium-copper-magnesium alloy suitable for aerospace applications. Such alloy comprises essentially the following composition (in weight %):
Cu4.85-5.3 Mg0.5-1.0Mn0.4-0.8Ag0.2-0.8Zr0.05-0.25Fe≦0.10Si ≦0.10,
the balance aluminum, incidental elements and impurities. The alloy is substantially vanadium-free and lithium-free wherein the non-presence of vanadium has been reported as being advantageous for the observed typical strength values. At the same time the addition of silver has been reported as to enhance the achievable strength levels of T6-type tempers. However, such alloy has the disadvantage that it is quite expensive for applications such as structural members of an aircraft even though it is reported to be suitable for higher temperature applications such as aircraft disc rotors, calipers, brake drums or other high temperature vehicular applications.